Set up the needle
Hold the needle by the shaft or place it on a stable surface with the eye accessible. Keep the point away from your fingers and make sure the eye is free of old fibres.
Push the threader wire gently through the eye. If it will not pass, stop and check the eye rather than bending the wire against resistance.
Load and draw back
Place the trimmed sashiko thread through the wire loop with only enough overlap to hold it. A large loop or folded bundle can jam as you pull it through.
Draw the wire back in the same direction it entered, keeping the thread aligned with the eye. Move slowly if the thread is full or loosely twisted.
Finish without a kink
Once the thread is through, remove the wire and smooth the strand. Check that no fibres were cut or pulled out at the eye before starting the first stitch.
If the threader catches, release it rather than yanking. Trim a fresh end or choose a better-matched needle and try again.